We review the synthesis of semiconductor nanocrystals/colloidal quantum dots in organic solvents with special emphasis on earth-abundant and toxic heavy metal free compounds. Following the Introduction, section 2 defines the terms related to the toxicity of nanocrystals and gives a comprehensive overview on toxicity studies concerning all types of quantum dots. Section 3 aims at providing the reader with the basic concepts of nanocrystal synthesis. It starts with the concepts currently used to describe the nucleation and growth of monodisperse particles and next takes a closer look at the chemistry of the inorganic core and its interactions with surface ligands. Section 4 reviews in more detail the synthesis of different families of semiconductor nanocrystals, namely elemental group IV compounds (carbon nanodots, Si, Ge), III-V compounds (e.g., InP, InAs), and binary and multinary metal chalcogenides. Finally, the authors' view on the perspectives in this field is given.

Ternary metal chalcogenide nanocrystals (NCs) offer exciting opportunities as novel materials to be explored on the nanoscale showing optoelectronic properties tunable with size and composition. CuInS (CIS) NCs are the most widely studied representatives of this family as they can be easily prepared with good size control and in high yield by reacting the metal precursors (copper iodide and indium acetate) in dodecanethiol (DDT). Despite the widespread use of this synthesis method, both the reaction mechanism and the surface state of the obtained NCs remain elusive. Here, we perform in situ X-ray diffraction using synchrotron radiation to monitor the pre- and postnucleation stages of the formation of CIS NCs. SAXS measurements show that the reaction intermediate formed at 100 °C presents a periodic lamellar structure with a characteristic spacing of 34.9 Å. WAXS measurements performed after nucleation of the CIS NCs at 230 °C demonstrate that their growth kinetics depend on the degree of precursor conversion achieved in the initial stage at 100 °C. NC formation requires the cleavage of S-C bonds. We reveal by means of combined 1D and 2D proton and carbon NMR analyses that the generated dodecyl radicals lead to the formation of a new thioether species R-S-R. The latter is part of a ligand double layer, which consists of dynamically bound dodecanethiolate ligands as well as of head-to-tail bound R-S-R molecules. This ligand double layer and a high ligand density (3.6 DDT molecules per nm) are at the origin of the apparent difficulty to functionalize the surface of CIS NCs obtained with the DDT method.

In the search for low‐cost thermoelectric materials operating near room temperature, the potential of chalcopyrite (CuFeS2) nanocrystals is explored. Their colloidal synthesis is optimized to achieve around 40 nm sized nanocrystals with the goal to effectively reduce thermal conductivity via phonon scattering while maintaining high electrical conductivity. EDX and XPS analyses reveal that the nanocrystals are intrinsically nanostructured with a radial compositional gradient. Three strategies are explored to optimize the thermoelectric properties: i) Intrinsic doping by varying the Cu : Fe ratio. However, the effect of this variation is overcompensated by a global sulfur deficiency, making the chalcopyrite nanocrystals n‐type. A high Seebeck coefficient, S, up to −380 μV/K is obtained, while the figure of merit remains comparably low (ZT=0.07 at 400 °C) because of low electrical conductivity σ. ii) Removal of the native, insulating ligands by exchange with potassium selenide. This results in a better trade‐off between S and σ and hence a strongly improved ZT (0.18 at 400 °C). iii) Extrinsic doping via intimate mixture of chalcopyrite nanocrystals with metal nanoparticles. Sn (3 wt %) or Ag (16 wt %) nanoparticles give the best results (ZT=0.16 at 400 °C), inducing the concomitant reduction of thermal conductivity κ and increase of σ.

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tert-butylthiol (tBuSH) is used as the sulfur source, surface ligand and co-solvent in the synthesis of CuInS2 nanocrystals (NCs). The presented method gives direct access to short-ligand-capped NCs without post-synthetic ligand exchange. The obtained 5 nm CuInS2 NCs crystallize in the cubic sphalerite phase with space group F-43m and a lattice parameter a=5.65 Å. Their comparably large optical and electrochemical band gap of 2.6-2.7 eV is attributed to iodine incorporation into the crystal structure as reflected by the composition Cu1.04 In0.96 S1.84 I0.62 determined by EDX. Conductivity measurements on thin films of the tBuSH-capped NCs result in a value of 2.5(.) 10(-2)  S m(-1) , which represents an increase by a factor of 400 compared to established dodecanethiol-capped CuInS2 NCs.

Permanently renewing security and tracking technologies is mandatory to fight the worldwide growing problem of conterfeiting, doped by the development of e-commerce and the consequences of social/sanitary crisis. In this context, a versatile microimprinting technology is proposed herein to directly tag the surface of an object of interest. The resulting photoluminescent micropatterned tags, based on an adhesive epoxy loaded with quantum dots, emit a specific optical signal when excited with UV light. The design of the tags is fully tunable and can authenticate a product and/or store secured traceability data with a lateral resolution of patterns down to 5 μm. Obtained tags are photostable up to 100°C. This technology is compatible with numerous surface configurations (flat/curved, horizontal/vertical, etc.), substrate materials (polymer, metal, glass, fibers, etc.), and various types of quantum dots or nano-objects. It can be used manually or implemented in a production line. Demonstration of the technical capabilities is made with various types of quantum dots. The key experimental parameters influencing the photoluminescence signal and the tag signal contrast are investigated in the case of InP@ZnS ones. As a proof of concept, three concrete tagging use cases are finally illustrated: electronic components, textiles, and ammunition for firearms.

Among inorganic semiconductors, ternary and quaternary chalcogenides have attracted interest as light absorbers in photovoltaic applications. Cu 2 ZnSnS 4 (CZTS) has drown considerable attention as it has band‐gap suitable for solar‐harvesting applications, it shows p‐type conductivity and a high absorption coefficient. Moreover it only consists of inexpensive, non‐toxic and earth‐abundant materials. Synthesis by wet‐chemical methods are promising alternatives to physical deposition processes, as more easily implemented and cheaper. One of the challenges in the synthesis of colloidal CZTS nanocrystals is the control of internal structure and composition, which influence significantly their optoelectronic properties [1]. In this presentation we show the evidence of cation ordering in CZTS structure thanks to STEM HAADF imaging and we analyze nanocrystals homogeneity and composition by STEM EDX. CZTS nanocrystals were syntesized following an heating‐up method [2]. The first stage of the synthesis consists in a 30 minutes pre‐heating at 110°C of the organometallic precursors mixed in oleylamine. Then, CZTS nCs are obtained by increasing the mixing temperature up to 280°C and keeping it constant for one hour. The presence of long‐chained organic ligands passivating the surface of nanocrystals is fundamental for avoiding agglomeration in solution phase, it allows a slow and controlled growth; nevertheless it is detrimental for application in devices and for electron microscopy studies, in particular in spectroscopy (where contamination is critical). By drop‐casting the sample on graphene membranes, we could test the influence of several purification strategies. Thanks to the low‐contrast support we could image the unwanted parasitic residuals. In particular we proved the efficiency of solvent/antisolvent chloroform/aceton + acetic acid dispersion cycles [3]. HRTEM characterization was performed ex‐situ. HRTEM and STEM‐HAADF images were used to measure size dispersion of the nanocrystals. HRSTEM‐HAADF is sensible to chemical contrast, the signal being dependent on the atomic number Z; it is then possible to observe the sites occupied by the heavier atoms (Sn) in the structure, and distinguish then between kesterite (space group I‐4) or stannite (space group I‐42m) and pre‐mixed Cu‐Au (PMCA, space group P‐42m) structures, which show different characteristic “bright” motifs. The latter (PCMA) structure was the one found when nanocrystals were showing the good direction for phase identification (111). HRSTEM‐HAADF experimental images were compared with simulated ones obtained by multislice method and thermal diffusion scattering approximation [4]. STEM‐EDX was carried out on a dedicated FEI Themis with SuperX detector, in order to ensure chemical homogeneity between nanocrystals and inside a single crystal. Spectra were analyzed and quantified using Bruker Esprit 1.9 software.An overview of the nucleation and growing process was obtained by in‐situ Wide‐Angle X‐ray Scattering (WAXS) and Small‐Angle X‐ray Scattering (SAXS), performed on the ID01 beamline at the European Synchrotron Radiation Facility.

The Inside Cover picture shows short ligand‐capped CuInS2 nanocrystals (red), which are synthesized by using tert‐buthylthiol as the sulfur source, ligand, and co‐solvent. They show a 400‐fold increase in conductivity compared to nanocrystals capped with dodecanethiol (blue).More details can be found in the Communication by P. Reiss and co‐workers on page 654 in Issue 5, 2016 (DOI: 10.1002/cphc.201500800).